An apparatus of the above kind is known which comprises a hand-held and/or tripod mountable master and slave units. In use of the apparatus the slave unit is placed at a location whose distance from the master unit is to be measured. Each unit has a respective transmitter circuit for transmitting towards the other unit a burst of ultrasound and each unit has a respective receiver circuit to detect the burst of ultrasound transmitted by the other unit.
The measuring process starts when a `GO` or start button is pressed which causes the master unit to transmit a short ultrasonic burst to the slave unit, which responds to the detection of such burst by transmitting a short ultrasonic return burst back to the master unit. The master unit determines the distance between the master and slave units based upon the time elapsed between the transmission of the outward burst by the master unit and the detection of the return burst by the master unit.
This elapsed time (which represents double the desired distance) is used, according to a well known equation, including air temperature which is sampled and supplied as an input quantity, to calculate the distance travelled in air equivalent to half the elapsed time, which is then displayed on a suitable display.
This known apparatus has several major disadvantages, and the present invention and preferred embodiment seek to mitigate these disadvantages.
The prior art apparatus referred to above uses two different ultrasonic transmission frequencies for the master unit and slave unit, 25 KHz for the master unit and 40 KHz for the slave unit. The reason for this is to attempt to make the master unit receiver circuit immune to unwanted echoes of its own outward burst, so that only a return burst from the slave unit is detected.
This works quite well in open spaces. However, in a cluttered environment, and despite the different return frequency, the master unit receiver circuit will nevertheless respond to high level echoes of its own outward burst bouncing off nearby objects, thereby giving false readings.
To avoid this one would need highly sophisticated and hence expensive filters in the master unit receiver circuit to reliably discriminate between e.g. a `faint` 40 KHz return burst from the slave unit and a `loud` 25 KHz echo from a nearby object.
Instead, however, the prior art apparatus overcomes the problem by taking up to 25 successive measurements, and only displays a reading when four measurements coincide. This can take several seconds per reading, and the need to have four coincident measurements militates against the user holding a master unit and moving slowly but continuously towards or away from a static slave unit with the `GO` button continuously pressed in order to establish a desired distance from the slave unit.